Stephen G. Anderson

Fundamentals of tandem mass spectrometry: a dynamics study of simple C−C bond cleavage in collision-activated dissociation of polyatomic ions at low energy

The loss of methyl radical in collision-activated dissociation (CAD) of acetone and propane molecular ions has been studied at low energy using a tandem hybrid mass spectrometer. Although the two processes are very similar chemically and energetically, very different dynamical features are observed. Acetyl ions from acetone ion are predominantly backward-scattered, with intensity maxima lying inside and outside the elastic scattering circle, confirming our previous observation that electronically excited states are important in low-energy acetone CAD. Ethyl ions from propane ion show a forward-scattered peak maximum at a nonzero scattering angle, which is consistent with generally accepted models for vibrational excitation and redistribution of energy before dissociation. Both processes demonstrate that CAD at low energy proceeds via small-impact-parameter collisions with momentum transfer. Comparison of the present results with higher energy CAD dynamics studies and earlier work leads to some tentative general conclusions about energy transfer in these processes.

A hybrid tandem supersonic beam mass spectrometer for the study of collision-induced dissociation of ions in the energy range <1 to 3000 eV

Abstract A hybrid tandem mass spectrometer has been constructed to study the dynamics of collision-induced dissociation processes in the energy range of less than 1 eV to several keV. A mass- and energy-analyzed high-energy ion beam is decelerated to low energies by a series of cylindrical and rectangular tube lenses. The decelerated ion beam collides with a supersonic neutral beam at right angles. Energy and mass analysis of the fragment ions is performed by a novel hemispherical energy analyzer followed by a quadrupole mass filter. The detector system can be rotated about the collision center to provide angular analysis of scattered fragment ions. Ion beams of moderate intensity have been obtained in the entire energy range, it is especially significant that good intensity is obtained in the lower threshold energy range of 0.2–5 eV. The performance of the instrument has been evaluated by an extensive series of ion transmission, focus, and energy measurements and by comparison of CID results with known mstastable and collision-induced dissociation processes.

Collision-induced dissociation reaction dynamics of the acetone molecular ion

Abstract The collision-induced dissociation (CID) reaction CH3COCH+3 + Ar → CH3CO+ + CH3 + Ar has been studied with a tandem hybrid mass spectrometer (a high-resolution mass spectrometer coupled with a supersonic neutral beam, which uses post-collision energy and mass analysis) at ion laboratory energies in the range 4.5–300 eV. Kinetic energy and angular distributions of the fragment CH3CO+ ions were measured and scattering contour diagrams for this process were constructed. The results show that the dissociation proceeds via low-impact parameter collisions with extensive momentum transfer. At 4.5 eV, the product is primarily backward scattered with some intensity at the center-of-mass (completely inelastic collision). At higher energies the product ion is progressively more forward scattered but over the energy range investigated the maximum intensity never shifts to 0°. Collectively, these results suggest that impulsive mechanisms dominate CID reactions at all energy ranges for this ion and that the dominant mechanism is kinematically different at low and high energy.

Collision-induced dissociation of the propane molecular ion

Abstract A crossed-beam tandem hybrid mass spectrometer equipped with a supersonic neutral beam source was used to investigate collision-induced dissociation (CID) of the propane molecular ion at a series of relative collision energies from 1.7–449 eV. The competing processes of elimination of methane and loss of methyl radical were studied using beams of helium and argon neutrals. The dynamics of both dissociation processes change relatively little with increasing collision energy, demonstrating that the CID mechanism does not change with collision energy over a very broad range. The most probable energy transfer into the ion increased only slightly with collision energy. The dynamics are consistent with previous conclusions that internal energy is randomized amongst internl degrees of freedom before dissociation. The intensity maxima at all energies occurred at non-zero scattering angles in the center-of-mass reference frame; scattering angles decrease with increasing collision energy but never decreased to zero. This implies that the CID mechanism for propane ion is dominated by small impact parameter collisions with large angular momentum exchange at all energies.

Observation of fine‐structure transitions in argon charge transfer at low energies using a crossed‐beam technique

The crossed‐beam method was used to investigate the charge–transfer reaction of Ar+(2P3/2) with Ar(1S0) as a function of collision energy over the range of 2–100 eV (center of mass). Both the exactly resonant channel and the endoergic fine‐structure transition with ΔJ=1 were detected and the cross section ratio σ3/2→1/2/(σ3/2→1/2+σ3/2→3/2) was found first to increase with increasing collision energy to a maximum value of about 0.5 at 30 eV and then decrease at higher energy. The relative abundance of the fine‐structure transition channel at low energy is much higher than that predicted by the accepted general theory for these processes. It is suggested that a curve‐crossing mechanism not previously considered and the perturbation of trajectories at low energies resulting from the strongly bound Ar+2 intermediate may be responsible for the unexpected observations.

Observation of fine-structure transitions in rare gas charge transfer at surprisingly low energies using a crossed-molecular beam technique

Abstract The crossed-beam method was used to investigate the charge-transfer reaction of Kr+(2P 3 2 ) with Kr(1S0), using a nearly pure Kr+(2P 3 2 ) ion beam. The resonant charge-transfer reaction is well described by a standard theoretical rectilinear-trajectory model. The endoergic fine-structure transition with ΔJ = 1 was also detected. This product is scattered at a definite angle, suggesting a short-range interaction which selects a particular impact parameter. Both channels are in general accord with accepted theories for ion/atom charge exchange, but the energy range at which the endothermic channel is observed is much lower than predicted.

A high transmission hemispherical energy analyzer for ion spectrometry

A hemispherical energy analyzer was constructed by using a novel approach to control the fringing electrostatic field. It provides several properties useful in ion spectrometers: namely, rather simple fabrication and compact size, high transmission efficiency at moderate resolution, and the capability to adjust resolution by changing the intersphere potentials. A computer program was developed to evaluate ion trajectories through the hemispherical analyzer. Data obtained from the trajectories were used to predict the characteristics of the analyzer. Experiments performed to determine the kinetic energy dependence of the absolute transmission and the resolution functions are in accord with theoretical calculations.